The present invention relates to an ohmic electrode suitable for use in compound semiconductor devices.
Conventionally, an ohmic electrode of a gallium arsenide (GaAs)-based compound semiconductor device is formed on a GaAs-based semiconductor layer or a GaAs substrate by subjecting a Ni/Au--Ge metallic layer to alloying treatment. More specifically, an ohmic electrode can be formed, for example, by depositing an Au-(12 wt.%)Ge layer, then a nickel layer thereon by means of vacuum deposition process, and alloying the resulting metallic layer by heating it at a temperature range of from 400.degree. to 450.degree. C. for a duration of 1 to 2 minutes.
In the alloying treatment, the Au--Ge alloy is molten to form a liquid phase at a temperature not lower than the eutectic point of 356.degree. C. Because the surface of an n-type GaAs compound semiconductor layer is unsaturated, gallium and arsenic dissolve into the Au--Ge melt during the alloying treatment. Thus is formed an alloy layer comprising, in addition to an Au--Ga alloy, a Ni--As--Ge alloy which results from arsenic, nickel, and germanium that are present in excess. During lowering the temperature, that is, during the re-growth of GaAs, germanium is incorporated into GaAs to occupy the vacancy sites of gallium (Ga) to form an n.sup.+ layer. Nickel plays a role of absorbing gold and germanium to form a film of Ni--Au--Ge alloy, and of preventing boring up of Au--Ge alloy. An electrode material layer is deposited thereafter on the alloy layer, and is etched into a desired shape to obtain a complete ohmic electrode.
An ohmic contact between the metallic Ni/Au--Ge layer and the base layer, i.e., the GaAs-based semiconductor layer or the GaAs-based substrate, can be formed by alloying the metallic Ni/Au--Ge layer as described above. However, the heat treatment for the alloying must be performed at a temperature in the range of from 400.degree. to 450.degree. C. for a duration of 1 to 2 minutes. The temperature of alloying is sufficiently lower than, for example, the crystal growth temperature for GaAs considering that an MOCVD process is effected at about 800.degree. C. Hence, alloying was believed to be free of problems, i.e., to have no influence on ion implantation and other processes for fabricating compound semiconductor devices.
Considering the process for growing a semiconductor layer of a compound consisting of a Group II and a Group VI element of periodic table (hereinafter referred to simply as a "Group II-VI compound semiconductor") on an n-type GaAs substrate, however, the temperature during the process is maintained at 300.degree. C. or lower. Therefore, problems are encountered in forming an ohmic electrode on the back of an n-type GaAs substrate, because crystal defects and the like generate on the Group II-VI compound semiconductor layer due to the heat treatment, and the crystallinity of the Group II-VI compound semiconductor layer is impaired unfavorably by these defects.
An organic thin film such as of polyimide is used in an optoelectronic integrated device comprising thereon a laser element and an optical waveguide. The organic thin films cannot resist to a heat treatment performed at such a high temperature of 400.degree. C. or even higher. Moreover, such an alloying treatment is problematic in that it generates thermal stress on optical waveguides.
The aforementioned problems can be overcome if the heat treatment is effected at 350.degree. C. or lower. However, considering the metallic layers such as Ni/Au--Ge commonly used at present, no favorable ohmic contact can be realized between the electrode and the compound semiconductor layer or the compound semiconductor substrate by means of a heat treatment effected at a temperature of 350.degree. C. or lower.
In case of a Pd/Ge based metallic layer, T. C. Shen and others report in "Recent Development in ohmic contact for III-V compound semiconductor", in J. Vac. Sci. Technol. B 10(5), September/October (1992) pp. 2113-32 that a favorable ohmic contact is implemented in the temperature range of from 325.degree. to 375.degree. C. However, Pd is thermally unstable, and the alloying process requires an extremely long duration of 30 minutes.